1. Field of the Invention
Rotary drum filters are used in the pulp and paper industry for filtering liquid out of pulp. Such devices are cylindrical in shape and are typically constructed of metallic and fiberglass materials. The outer drum surface acts as a filtering medium or structure, through which the liquid passes while the pulp remains on the surface of the structure as the drum rotates, partially submerged in a tank containing a paper pulp stock. In the manufacture of rotary drum filters, a large cylinder is structurally supported on the inside by flyrings and is closed on the ends by end supports and the outside surface of the cylinder is rarely true to the centerline of the drum. Longitudinal ribs or grids are installed on the outside surface, or deck of the cylinder in spaced relationship and extend along the length of the cylinder deck. Since the outer edges of the grids are not true to the centerline of the drum, the grids must be machined true to this centerline, a procedure which is very time consuming and costly. In the case of open-channel decks, after this machining is completed, backing wire grooves are spirally machined into the outer edges of the grids and backing wire is wrapped in the grooves to provide a base which supports a filter screen. In the case of corrugated anti-rewetting decks, a corrugated filter plate is installed after the grids are machined true to the drum centerline.
The grid extension technique and apparatus of this invention applies equally well to the rebuilding of open-channel rotary drums with deteriorated backing wire grooves, and construction of new drums. The present state of the art in constructing a grid system of uniform radius with respect to the centerline of the drum requires one of two basic procedures; either machine new backing wire grooves to the desired uniformity in the grids or weld a grid extension to each grid and machine new grooves in the the grid extensions to a true tolerance. Both solutions require valuable machine time and are expensive. The pin and groove method of installing grid extensions according to this invention minimizes the cost of reconstructing filter drums. This invention is applicable to open-channel decks and also includes anti-rewetting grid extension configurations which can be installed by the same pin and groove method. The procedure of this invention regarding existing drums which are ready to be refurbished is to remove the existing backing wire; machine spaced, circumferential slots in the existing grids; install single-leg, inside and outside grid extensions or anti-rewetting grid extensions provided with pin locators; and reinstall the drum backing wire. The anti-rewetting grid extension construction is useful in many applications to improve washing efficiency due to reduced air entrapment and reduction of rewetting in the filter cake at higher pulp production rates.
2. Description of the Prior Art
Rotary drum filters of the same general design as that disclosed in U.S. Pat. No. 3,150,082, dated Sept. 22, 1964, to John P. Rich, have long been known in the art. This patent details a rotary drum filter which includes a tank adapted to contain a mixture to be filtered, a cylindrical drum mounted for rotation in the tank and partially submerged in the mixture, and an improved valve means which is designed to reduce to negligible proportions any contained volume of liquid, particularly that volume adjacent the filter surface, where liquid might be held and contamination realized. U.S. Pat. No. 3,150,083, dated Sept. 22, 1964, to Oscar L. Nashua, entitled "Filters", discloses a rotary drum filter of the valveless vacuum type, wherein each backward-oriented passageway which drains a drum sector extends for the entire length of the filter drum, such that the filtrate may pass directly into and through the filtrate passageway without the necessity of movement in an axial direction along the length of the drum. The patent details a rotary drum filter which provides a maximum number of drum sectors, each with an independent passageway which provides an efficient shape, construction and arrangement of passageways from a mechanical and hydraulic efficiency standpoint. A "Center Valve Filter" is disclosed in U.S. Pat. No. 3,327,863, dated June 27, 1967, to Ralph W. Turner, Jr. The "Center Valve Filter" is designed to facilitate the venting of filtrate compartments in a novel manner directly from a valve structure located directly at the end of the filtrate compartment. This feature is said to provide a greatly improved efficiency of filtrate removal. U.S. Pat. No. 3,403,786, dated Oct. 1, 1968, to Oscar L. Nashua, entitled "Rotary Drum Filter" includes an improved rotary drum filter which is constructed for filtering a slurry such as the paper pulp stock employed in the manufacture of paper. The filter includes inner and outer drum members extending in concentric relationship, to provide a space between the drum members. A rolling means is disposed within the space between the drum members for supporting the outer drum member upon the inner drum member and dividing the space into a plurality of compartments. One of the drum members is constructed to collect filtered stock during rotation and an operating means is provided for rotating this drum member. The "Rotary Drum Filter" disclosed in U.S. Pat. No. 3,363,774, dated Jan. 16, 1968, to Oscar L. Nashua, discloses a drum filter having an axial end valve and containing compartments located on the circumference, which compartments are enclosed by cover plates. The cover plates are arranged to provide an inlet slot opening into each compartment located adjacent the trailing end of that compartment. Each conduit leading from a compartment to the axial end valve is positioned in a circumferential direction, such that a major portion of the volume contained in the conduit and the corresponding compartment rotates ahead of an angular plate extending through the compartment slot. Liquid contained in the conduit and in the subject compartment is substantially prevented by the cover plate from draining by gravity out of the slot throughout a substantial portion of the rotary downward travel of the filtrate compartment, from a position of maximum height in the rotary path of the filter drum. U.S. Pat. No. 3,530,992, dated Sept. 29, 1970, to Ralph W. Turner, Jr. and entitled "Rotary Drum Filter" is directed to a rotary drum filter which includes a rotatable drum having generally longitudinally extending filtrate compartments along the deck periphery. The filtrate compartments include juxtaposed open ends and the filter is caused to rotate through a first filter cycle during which sub-atmospheric pressure is momentarily applied through the open ends to the compartments for filter cake accretion, and a second cycle, during which the sub-atmospheric pressure is cut off from the compartments. The application of the sub-atmospheric pressure is controlled by a stationary valving assembly which includes plate elements resiliently biased into sliding engagement with the drum at the open ends of the filtrate compartments. U.S. Pat. No. 4,066,555, dated Jan. 3, 1978, to Joseph A. Pascale, entitled "Rotary Filter", includes an improved rotary filter which is cylindrical in shape and operates by vacuum suction to withdraw fluids from a substance exposed to the outer area of the device. The operation calls for drawing the fluids into the filter and pumping them to a suitable reservoir, and the system includes an inner cylindrical surface with a plurality of selectively spaced support structures extending longitudinally along the inner cylinder. The support structures are covered with a plastic covering which is adapted to receive a filtering medium forming the outer circumference of the rotary filter and in one embodiment, the plastic is grooved to receive a spirally-wound wire to further support the filtering medium. A "Method of Making a Rotary Drum Filter and Method of Making an Annular Valve Housing for Such a Drum Filter" is detailed in U.S. Pat. No. 4,419,165, dated Dec. 6, 1983, to Richard W. LaValley. The filter device which is detailed in this patent includes a plastic rotary drum filter which is constructed by applying a layer of plastic to the interior surface of a cylindrical mold, removing the mold, and applying at least one additional layer of plastic to the exposed exterior surface of the shell formed by the first layer. A plastic annular axial valve housing with plural valve chambers and a central valve-receiving space is mounted to a head assembly at the end of the shell. A tail assembly closes the opposite end of the shell to form the drum. Circumferentiallyspaced interior plastic bucket sections are positioned midway between the head and tail assemblies. Upon periodic application of a vacuum to the valve chambers, filtrate is delivered from the enterior of the drum through an associated bucket and plastic drainage conduit to a cooperating valve chamber. Plastic reinforcing rings strengthen the drum along the length thereof and in the region of the bucket section.
An object of this invention is to provide new and improved grid extensions for installation on the grids of conventional vacuum filters, which grid extensions are each so mounted as to be automatically situated at a true radius with respect to the theoretical centerline of the filter drum.
A still further object of this invention is to provide new and improved grid extensions which are each characterized by a single-leg grid extension provided with spaced pins for seating in grooves or slots provided in conventional single-leg or channel grids located on the filter drum deck, to automatically align and seat the pins in the grooves or slots and orient the grid extensions and back wire mounted on the grid extensions, to a common radius with respect to the theoretical centerline of the filter drum.
Yet another object of the invention is to provide new and improved anti-rewetting grid extensions for mounting on the grids of a vacuum filter, which anti-rewetting grid extensions are characterized by L and T-shaped grid extension plates provided with spaced pins for seating in grooves or slots provided in the grids to locate the anti-rewetting grid extensions at a common drum radius with respect to the theoretical centerline of the drum.
It is another object of this invention to provide a new and improved technique for repairing the grids of rotary drum vacuum filters without the necessity of machining the grids to a true radius with respect to the theoretical drum centerline.
Still another object of this invention is to provide a new and improved method for installing grid extensions on the existing grids of vacuum drum filters, which method includes the steps of providing spaced grooves or slots in the existing drum grids, such that the base of each of these grooves or slots corresponds to a common radius which is measured from the theoretical longitudinal centerline of the filter drum; inserting pins through multiple grid extensions at points on the grid extensions which correspond to the grooves or slots in the grids; and welding the pins in the grooves or slots to locate the grid extensions longitudinally along the existing grids at a second, larger common radius with respect to the theoretical longitudinal centerline of the filter drum.